Off-road bicycles (i.e., mountain bikes) are subjected to bumpy terrain that cause the chain to bounce around and sometimes derail from the front sprocket. Most off road bicycles have 3 front sprockets, (i.e., small, medium, and large sprockets), to provide a wide gear range to allow pedaling uphill and downhill. Three front sprockets, however, make it difficult to provide chain retention for rough conditions while still allowing shifting. For downhill racing, off-road bikes typically have just one front sprocket since they typically do not need to ride uphill. Downhill bikes are subjected to even more extreme conditions, which make the likelihood of chain derailment even greater.
With a single sprocket, chain retention is sometimes accomplished with circular plates on each side of the sprocket, and shallow-grooved pulleys or rollers at the chain entrance and exit (U.S. Pat. No. 5,725,450 to Huskey). The plates provide axial retention, and the pulleys provide chain retention in the radial direction. The chain is trapped on all sides. The circular plates also provide protection for the sprocket from impact damage. Another chain retention method involves deeply grooved pulleys at the chain entrance and exit from the sprocket. With this approach the chain is trapped by the flanges and grooves of the pulleys.
Both of these methods, however, do not allow the use of multiple front sprockets and/or shifting. The present invention allows the use of multiple front sprockets and/or shifting while providing positive chain retention at the chain exit from the sprocket. This invention is preferably used with a standard front derailleur, which shifts the chain on the front sprockets.